The present invention also concerns a printing machine suitable for carrying out the process according to the invention.
Printing processes and machines are known per se in the prior art. For example, U.S. Pat. No. 4,574,696, the content of which is incorporated by reference in the present application, discloses a rotary printing press for the simultaneous multicolour printing on both sides of a web or sheet. The advantage of this machine consists in that it permits exploiting simultaneously two totally different methods for printing in a single pass an offset image on one side and an “Orlof” image on the other side, thus offering the user for the first time the possibility of printing notably safety backgrounds on both sides of a bank note through two different methods; this increases the safety against forgery and in addition makes printing operations more economical because the user is not compelled to use two separate machines.
Another printing machine is known for example from U.S. Pat. No. 4,640,189, the content of which is incorporated by reference in the present application. This printing machine can print on both sides of a paper web either an image with juxtaposed colors by means of a typographic plate inked respectively by a collecting cylinder inked in turn by selective inking cylinders the number of which corresponds to the number of colors, or an image with superposed colors and designs by means of the plate cylinders substituted for the selective color inking cylinders and provided with printing plates inking a corresponding offset cylinder and of which the number corresponds to the number of colors and designs, or an image of each of the above-mentioned type. For this purpose, the machine comprises a first pair of blanket cylinders operating either as collecting cylinders each adapted to ink a cylinder carrying said typographic plate of which the image is transferred via an intermediate cylinder to another blanket cylinder of a second pair on the paper, or as offset cylinders contacting said other blanket cylinder, or operating one as collecting cylinder and the other as an offset cylinder. In all cases, the paper passes between the blanket cylinders of the second pair.
Another printing technique so called silk-screen printing is also known in the art. For example, U.S. Pat. No. 6,109,172, the content of which is incorporated by reference in the present application, discloses a silk-screen printing machine with a printing cylinder engaging two stencil cylinders for printing at least two non-overlapping areas in different colors using the one printing cylinder.
Document DE 197 03 312 discloses a printing machine in which the sheets are held by a chain gripper system while being printed. However, in this document, the speed of rotation of the printing cylinder is maintained constant.